Method for developing electrostatic latent images



Feb. 25, 1958 FAUsER r 2,824,813

METHOD FOR DEVELOPING ELECTROSTATIC LATENT IMAGES Filed May 12, 1952HIGH VOLTAGE SUPPLY G HIGH VOLTAGE SUPPLY R R E w @HL 7 0 ms Y ML. EEDAA N V L R IA U T s o F M Y G B F a H 2 2 States FDR DEVEL'GPINGELECTROSTATIC LATENT IMAGES 1 Claim. (Cl. 117-175) ltilETHiDl) Thisinvention relates in general to xerography and, in particular, to a newXerographic process.

According to the invention disclosed in Carlson U. S. Patent No.2,297,691, an electrostatic latent image is produced on aphotoconductive insulating surface in one of several ways such as byimposing a uniform electrostatic charge on the surface and exposing thecharged surface to an energizing radiation pattern, whereupon theexposed areas become selectively discharged. It has been usual totransform this electrostatic latent image to a visible image by treatingit with an electrostatically attractable material, such as a fine powderor mist, by which treatment the electrostatically attractable materialis selectively deposited on the surface bearing the electrostatic latentimage. The deposited image is then transferred to another surface orotherwise utilized to form a useful product, and the image surface iscleaned if necessary, recharged and again exposed to energizingradiation.

it has now been found that the Carlson process of electropl'iotography,since known as x'erography, can be modilied and improved by a step inwhich the electrostatic latent image induces an electrostatic image onanother surface such as, for example, an adjacent conductive surface,and the induced image is developed or otherwise usefully employed.

it is an additional object of the invention to provide a new process fordeveloping an electrostatic latent image from an insulating orphotoconductive insulating surface.

it is a further object of the invention to provide a process includingthe step of developing an induced electrostatic latent image.

it is a still further object of the invention to provide a xerographicprocess comprising forming an electrostatic latent image, inducing acorresponding latent image on an adjacent conductive surface anddeveloping said induced image on the conductive surface.

Other objects of the invention will in part be obvious and will in partbe apparent from the specification and from the drawings in which:

Figure 1 is a diagrammatic view of apparatus accord ing to oneembodiment of the invention;

Figure 2 is a diagrammatic view of apparatus according to anotherembodiment of the invention;

Figure 3 is an enlarged fragmentary diagrammatic view of an imagebearing member and an induced-image member, illustrating the behavior ofelectrostatically attractable particles therebetween; and

Figure 4 is a fragmentary diagrammatic view of a portion of a modifiedapparatus according to a modified embodiment of the apparatus of Figure2.

Figure l is illustrated simple apparatus adaptable to an improvedxerographic process according to this invention. Shown in the figure isan electroscopic material source such as, for example, a liquid mistnozzle, a conductive electrode 11, optionally mounted on support means12, and an image bearing member 13 also optionally mounted on supportmeans 14. The electrode 11 and image bearing member are closely spacedacross a development zone tent 'such as ground or space 15, and thenozzle the space, substantially A conductive charging nozzle 16 byinsulating is directed into and through parallel to members 11 and 13.ring 16 is mounted in front of supports 37, being positioned so that themist or spray emerging from the nozzle passes through the ring. The ringis conductively joined to a potential source 19, optionally at variablepotential. image member is connected to a potential source, potential;or in the case of an image mem' er consisting of a conductive backingand an insulating layer, the conductive backing is thus connected,optionally this connection being through support means 14. Similarly,electrode 11 is conductively connected to a potential source, forexample, potential source 20, optionally a source of a variablepotential.

The operation of this mechanism depends upon the nature of the imagebearing member 13 and varies with the intended end purpose. This will bedescribed with reference to a typical situation, it being understoodthat the specific instance is not a limiting one. The usual image memberis a plate or other member having a conductive backing and an insulatingor photo-conductive insulating layer. In the case of a photoconductiveinsulating layer, the layer may be charged, for example, to a positivepolarity of about to 500 volts and exposed to a light image whereupon itbecomes selectively conductive under the influence of the selectiveillumination. The electric charge thus is selectively dissipated,yielding a latent image having various charged areas having potentialsranging downward from the original or maximum potential. Such an imagemember is placed on support 14 and the conductive backing iselectrically grounded.

In operation, ring to is connected to a potential of opposite polarityto the image on member 13, in this case a negative potential, andelectrode 11 is connected to ground potential or a potential a few voltsfrom ground potential. Spray nozzle it} is then energized, producing aliquid spray that consists or" line droplets positively charged byinduction from charging ring 16. These charged droplets are directedinto development zone 15 where they are deposited on electrode 11 by theinduced electrostatic image thereon.

In i i ure 2, there is shown a copying mechanism employing theprinciples or" this invention. in the mechanism shown in Figure 2, t.ere is an image-bearing member or cylinder 30 which, along at least onesegment of its area, has an image surface comprising a conductivebacking member and a photoconductive insulating layer 32 therein. Thecylinder is rotatably mounted on an axle 33 and is electricallyconnected to a suitable potential source, such as ground, by means of aconductive connection 34.

Fositioned around the cylinder are several stations at which may becarried out the process steps according to electrophotography orxerography. Illustrate-d diagrammatically is a charging station 36 whichmay, in a simple form, comprise a corona discharge electrodeconductively connected to a high voltage source 37. At a subsequentposition in the direction of rotation of the cylinder is an exposingstation including, for example, a camera housing 39 and a lens 46. itwill be understood that this exposing station may comprise appropriatecontact exposing means or suitable lens or projection exposing meanswhich may operate in any desired manner such as, for example, by meansof slit projection or the like.

A second cylinder 42 is positioned closely adjacent to cylinder 3i? at asubsequent position or station around cylinder 3i) to define adeveloping zone in the form of a narrow gap between the two cylinders.This cylinder 42 is spaced from cylinder 3i? as closely as is consistentwith the process steps of xerography and preferably is maintained with afree space less than Va", and usually in the order of about A free spacetherebetween. it is to be noted that it is desirable to have as narrow aspace as possible between these two members, the minimum of the widthbeing set in part by the need to flow a suspension of finite sizedparticles therebetween and to avoid contact caused by vibration, etc.,during movement of the members. The two cylinders preferably arerotatably mounted and driven from a single power source at synchronizedspeeds so that the surface of the cylinders across the developing zoneare adapted to move in the same direction and at the same rate of speed.

An image receiving material such as, for example, a sheet or web 43 ismounted to be carried across the developing zone along the surface ofcylinder 42, preferably being carried thereacross from a sheet feedingroll If desired, the material may be collected on a sheet receiving roll45 or optionally may be ejected from the apparatus in the form of aprint-containing sheet which is, in effect, the finished print. it willbe apparent that this sheet or web 4-3 is spaced from the printingcylinder 3% by a very narrow air gap which acts as the developing zoneand is completely free from contact with the printing cylinder at alltimes.

Cylinder 42 is electrically connected to a potential source which,preferably, is maintained at variable or adjustable potential such as,for example, a potentiometer 47 which may operate between a battery 48or other potential source and ground potential. In this manner, thecylinder 42 acts as a counter-electrode with reference to the printingcylinder 30 and is acted upon by a field of force determined by theelectrical potential or poten-v tial pattern on the adjacent surface ofcylinder Stl. it is at once apparent that a corresponding potentialpattern is induced on the surface of cylinder 42 at the development zonein the same manner as such pattern is induced on electrode 11 of Figurel, and accordingly, this cylin er 42 is analagous in operation andfunction to that electrode.

As illustrated in Figure 2, a suitable heating coil 49 may desirably bepositioned within cylinder 42. whereby the surface thereof may be heatedfor the purpose of fusing a thermoplastic material at the outer surfaceof the cylinder, or other means, as desired, may be employed to fix theimage to sheet 43.

Positioned to operate into the developing zone between cylinders 30 and42 is a source of a charged cloud or mist of finely divided particlessuch as, for example, a source of an electrostatically charged powdercloud. One form of such device is illustrated in Figure 2 and comprises,generally a cloud generating means 51, a cloud charging means 52 and aguide zone 53 which operates to direct the charge to the desiredposition. The cloud generating member comprises a container 54 having aninlet tube 55 connected to gas supply source such as an air source (notshown) and terminating within the chamber in a ring manifold 56. Themanifold has a plurality of upwardly aimed openings through which aircan be directed upwardly within the container. A mass of finely dividedpowder 57 is within the container, covering the manifold and preferablyabout half filling the volume of the con tainer.

A guide tube 58 leads from the upper or free space of the cloudgenerating means to the cloud charging means. Optionally this guide tubeis connected to the cloud charging means by an insulating sleeve 59.

The cloud charging means 52 consists of a conductive casing or electrode69 having mounted therein a corona electrode or needle 61 with aconductive support and lead 62 extending through casing 6i) and beingelectrically connected to a suitable potential, such as ground. Aninsulating block 63 preferably is placed in the casing wall to receivesupport 62 and hold the corona needle in position within the casing. Thecasing or electrode so is mounted at its other end to the guide member53 preferably by means of insulating washer 63. By means of insulatingwasher 63 and insulating sleeve 59 the casing for electrode 60 isinsulated from the rest of the mechanism and may be maintained at adesired potential independently of the potential or potentials of otherand adjacent mechanism. A conductive lead 64 electrically connects theelectrode 60 to a positive polarity high voltage source which may, forexample, be the same high voltage source 37 as employed for coronaelectrode 36. The general arrangement of the powder cloud generating andcharging means is disclosed and claimed in copending application SerialNo. 244,556.

While there is substantially no deposition of the charged powder cloudon the image-bearing cylinder 39, it is preferred to have a cleaningstation at one point along the cylinder which cleaning station may be arotating brush 66 or other cleaning means adapted to remove developingmaterial which from time to time will be deposited thereon in smallquantities, or may, if desired, be a means for cascading thereacross acleaning material such as for example, a granular material as shown inCopley U. S. 2,484,782.

in use and operation the mechanism of Figure 2 is employed as follows:

The two cylinders 30 and 42 are driven at a synchronized speed at such arate as may be desired. The high voltage source 37 is activated wherebya positive corona discharge is energized at electrode 36 and a negativecorona discharge is energized at electrode 61. A light pattern or imageis focused on the surface of cylinder through lens 40. In this mannerthe conventional functions of xerography are considered to occur oncylinder 3ft wherein an electrostatic charge is imposed onphotocon-ductive layer 32 by electrode 36 and the charge is selectivelydissipated between action of the light pattern from lens 46. Theresulting electrostatic charge pattern is carried around cylinder 39 tothe development zone where it induces a corresponding pattern on thesurface of cylinder 42. The charge patterns thus act to create a fieldof force across the developing zone which in turn deposits chargedparticles at the zone as disclosed in connection with Figure 1.

Along with the operation thus described, the powder cloud generatingmeans is activated by means of a gas passed through tube 55 whichcarries a powder cloud through charging means 52 and then through guidemeans 53. This cloud of charged powder particles emerging from guidemeans 53 into the developing zone is thereby caused to deposit on thesheet or web 43 which has been interposed between the developing zoneand the counter-electrode 42. This sheet then bearing the powder imageis carried by cylinder or electrode 42 out of the developing zone tocollecting roll 4-5. If desired, heating coils 49 are operatedpreferably with a warm up period whereby the powder image, ifthermoplastic, is fused or fixed on the sheet immediately upondeposition thereof. In this manner, the sheet or web collected on roll45 is the developed and fixed xerographic print.

It will be observed that the mechanism and apparatus shown in Figure 2yields a positively charged image on cylinder 30 and a negativelycharged powder. Since the induced image on cylinder 42 will be ofpolarity opposite to the image on cylinder 30, this will cause formationof a photographically negative or reversed image on the print receivingmaterial. A direct positive image will be formed when the electricalconnections to the charging means 52 are reversed to give positivepolarity corona therein.

The operation of image member 13 and electrode 11 to induce an image onelectrode 11 is best understood with relation to Figure 3, it beingunderstood that essentially the same mode of operation applies to themechanism of Figure 2. Image member 13 consisting of an insulating orphotoconductive insulating layer 21 on backing member 21a has anelectrostatic charge pattern or latent image as indicated by the plusmarks 22. Electrode 11, being spaced closely adjacent thereto acts as acomplementing condenser plate across the air gap or development zone,and lines of force 23 join the positive polarity image on member 13 toan induced negative polarity image on conductive electrode 11, the imagebeing indicated by negative mark 24. Positively charged particles fromnozzle 10 (Fig. 1) or nozzle 53 (Fig. 2) are propelled in the directionsindicated by the arrows, these directions being toward electrode 11where the induced charge exists and through the development zoneparallel with electrode 11 by the spraying force of nozzle 10 where noinduced image exists. It will be realized, of course, that in practicaloperation the deposition of particles on the induced image will seldomfollow exactly the theoretical pattern of the induced image because ofspace charge and like factors, and for this reason it may be generallydesirable to bias electrode 11 at a few volts potential, usually of thesame polarity as the charge on the particles. In spite of the fact thatan overall or average potential may be applied to electrode 11, it is tobe noted that the induced electrostatic image is superposed on theaverage charge potential, so that the electrode, although conductive,may at points repel charged particles while at other points itnevertheless attracts these same particles.

This process of developing the induced image is continued until thecharge on the deposited particles substantially neutralizes the inducedimage charge, at which time the deposited particles form a visible imagewhich is a mirror reproduction of the original electrostatic latentimage on member 13. The deposited visible image is then utilized, forexample, by transfer to a transfer surface by adhesive transfer,electrostatic transfer or other method, and optionally is fixed thereon.Alternatively, the electrode 11 may be a consumable conductive materialof color contrasting to the deposited image, and the image may be fixeddirectly thereon to form a xerographic print; or the image may bedeposited on a sheet or web held against electrode 11, the image beingselffixing or subsequently fixed.

It is to be observed that the original electrostatic latent image onmember 13 is substantially unaffected by the development process and isreusable to induce another image on the same or another electrode placedadjacent thereto. Thus, a single electrostatic image can be utilized forthe deposition of several successive visible images, being re-used untilit is erased by suitable methods such as re-charging or, in the case ofa photoconductive member, also by exposure to light.

Within the scope of the invention various effects of photographic toneand quality can be achieved by varying the potential on electrode 11 or42, either by variation from one print to another or by variation duringthe deposition of the particles on the induced image. Thus, by suitablebiasing of the conductive member 11 or 42, particle deposition can berestricted to the intense portions of the induced image or can be causedto occur on background portions in addition to image portions of thearea. Thus, for example, using the polarities shown in Figure 3,negative bias of the electrode 11 will cause overall deposition ofparticles even on background areas while substantial positive bias willcause blanking out of less intense areas.

It is to be understood that numerous variations and modifications may bemade within the scope of the present invention, and that the inventionis not to be limited to the specific embodiment described, but is to berestricted only to the scope of the appended claim.

What is claimed is:

The method of developing by making visible charge gradients induced onan equipotential conducting surface across a free space from an actualelectrostatic latent image on an electrostatic latent image bearingmember said method comprising, positioning an equipotential conductingsurface closely adjacent to an actual electrostatic latent image bearingmember with a small free space between said equipotential conductingsurface and said image bearing member, positioning an image receivingweb against said equipotential surface and between said equipotentialsurface and said image bearing member, said image bearing membercarrying on its surface an actual electrostatic latent image comprisingin image configuration electrostatic charges all of the same firstpolarity in respect to the equipotential surface, applying a potentialto said equipotential conductive surface opposite in polarity withrespect to the polarity of the electrostatic latent image and at apotential difference in respect to the highest potential of theelectrostatic image substantially equal to the highest potential of theelectrostatic image whereby charges opposite in polarity to the actualelectrostatic image are induced on said equipotential surface inconfiguration conforming with the actual electrostatic latent image,directing a suspension of electrostatically charged particles into thespace between said image bearing member and said image receivingmaterial, said particles being electrostatically charged to the samepolarity as the actual electrostatic latent image on the electrostaticlatent image bearing member, and developing by making visible the chargegradients by deposition of the charged particles on the image receivingweb through their attraction to said equipotential surface inconfiguration conforming with the actual electrostatic latent image.

References Cited in the file of this patent UNITED STATES PATENTS2,191,827 Benner et al. Feb. 27, 1940 2,233,037 Smith Feb. 25, 19412,287,837 Smyser June 30, 1942 2,376,922 King May 29, 1945 2,509,276Ransburg et al. Mar. 30, 1950 2,551,582 Carlson May 8, 1951 2,558,900Hooper July 3, 1951 2,573,881 Walkup et al. Nov. 6, 1951 2,576,047Schalfert Nov. 20, 1951 2,590,534 Hampe Mar. 25, 1952 2,633,796 PethickApr. 7, 1953 2,701,764 Carlson Feb. 8, 1955 FOREIGN PATENTS 605,979Great Britain Aug. 4, 1948

